1. Field of the Invention
The invention relates to an electroless deposition of film on molybdenum coated glass or other conducting substrates. The process does not need any external current or voltage source for the specific deposition of Cu(In,Ga)--(Se,S).sub.2 materials, and uses Fe or Zn, or Al for the source of counterelectrodes to initiate the electroless deposition of film, which is used for making photovoltaic devices.
2. The Prior Art
Thin-films of copper-indium-diselenide (CuInSe.sub.2), copper-gallium-diselenide (CuGaSe.sub.2) and copper-indium-gallium-diselenide (CuIn.sub.1-x Ga.sub.x Se.sub.2), all of which are sometimes generically referred to as Cu(In,Ga)Se.sub.2, have become the subject of considerable interest and study for semiconductor devices. Sulphur can also be used, and sometimes is, substituted for selenium, and the compound is sometimes also referred to even more generically as Cu(In,Ga)--(Se,S).sub.2 to comprise all of those possible combinations. These compounds are of particular interest for photovoltaic device or solar cell absorber applications because of solar energy to electrical energy conversion efficiencies that have been shown to exceed seventeen percent (17%) in active areas and to approach seventeen percent (17%) in total areas, which is high for current state-of-the-art solar cell technologies. It has been generally believed by persons skilled in this art that the best electronic device properties, and thus the best conversion efficiencies, are obtained when the mole percent of copper is about equal to the mole percent of the indium, the gallium, or the combination of the indium and gallium in the Cu(In,Ga)Se.sub.2 compound or alloy. The selenium content will not generally be important to the electronic properties of the semiconductor if the growth conditions supply sufficient selenium so that it comprises about fifty atomic percent (50 at. %) of the Cu(In,Ga)Se.sub.2 compound to form the desired crystal lattice structures.
A physical vapor deposition recrystallization method for selenization of thin-film Cu(In,Ga)Se.sub.2 is disclosed in U.S. Pat. No. 5,436,204, and entails depositing thin-film metal precursors Cu+(In,Ga) in a Cu-rich ratio of Cu/(In,Ga)&gt;1 on a substrate, annealing the precursors at a moderate temperature (about 450.degree. C.) in the presence of a Se overpressure to form thin-film Cu(In,Ga)Se.sub.2 :Cu.sub.x Se phase-separated mixtures, adding an (In,Ga) vapor exposure to the thin-film phase-separated mixtures in the Se overpressure while ramping the temperature up from the moderate temperature to a higher recrystallization temperature (about 550.degree. C.), maintaining the thin-film in the Se overpressure at the higher recrystallization temperature for a period of time to allow the Cu.sub.x Se and In,Ga+Se to form a slightly Cu-poor thin-film Cu.sub.x (In,Ga).sub.y Se.sub.z compound, and then ramping down the temperature of the thin-film while maintaining the Se overpressure.
U.S. Pat. No. 4,720,404 disclose the use of an aqueous alkaline bath for the adhesive chemical (electroless) deposition of copper, nickel, cobalt or their alloys with great purity, containing compounds of these metals, reducing agent, wetting agent, pH-regulating substance, stabilizer, inhibitor and complex former, characterized in that polyols and/or compounds of the biuret type are contained as complex former, as well as a method for the adhesive chemical deposition of the metals, employing this bath at a temperature from 5.degree. C. up to the boiling point of the bath, for the manufacture of printed circuits.
An electroless plating solution is disclosed in U.S. Pat. No. 5,158,604 in a process to plate copper and nickel. This process is accomplished by depositing metal onto a substrate which is catalytic to the electroless deposition of metal, and entails coating onto the substrate a layer of thixotropic viscous aqueous electroless plating solution comprising at least one ionic depositable metal species selected from groups 1B and 8 of the Periodic Chart of the Elements and chromium, at least one metal complexing agent present in molar excess of the depositable metal species, at least one reducing agent present in molar excess of the depositable metal species and sufficient thickener to provide a viscosity at 25.degree. C. which is in the range of 50 to 20,000 cp viscosity; wherein the viscosity of the solution is low enough to allow hydrogen gas generated by the deposition of metal to release from a catalytic substrate surface at a rate sufficient to allow the deposition of at least a 40 nanometer thick layer of metal onto a palladium catalyzed surface in less than 3 minutes.
U.S. Pat. No. 4,908,241 disclose a process for the currentless deposition of electropositive metal layers onto appropriate less electropositive metals by contacting an object to be coated with a coating bath, wherein a coating bath is used which contains a metal complex obtained by reacting a monovalent electropositive metal halide with a base, which is capable of complex formation with the electropositive metal, and a hydrohalic acid.
There is a need in the art of preparing Cu.sub.x In.sub.y Ga.sub.z Se.sub.n precursor films and powders for use in semiconductor device applications to prepare these films and powders by an electroless deposition process. The reason for need of an electroless deposition process is that such a process would provide:
a low cost, high rate process; PA1 a large area, continuous, multi-component, low temperature deposition method; PA1 deposition of films on a variety of shapes and forms (wires, tapes, coils, and cylinders); PA1 freedom from the requirement or need to use equipment; PA1 controlled deposition rates and effective material utilization; and PA1 minimum waste generation (solution can be recycled)